Fully bleached sulfite chemical pulp, a process for the production thereof and products derived therefrom

ABSTRACT

A process for preparing a bleached sulfite chemical pulp includes delignification of chips of a lignocellulosic material in a sulfite pulping process until the defibration point of the material is reached, and bleaching of the fibrous material thus obtained by a chlorine-free bleaching sequence which includes at least one first bleaching step with a chlorine-free oxidant in the presence of a base. The chemical pulp prepared according to this process and paper or nonwoven made from such pulp have a high degree of brightness (at least 83% ISO) and high strength.

[0001] The present invention relates to a completely bleached sulfitechemical pulp, a process for the production thereof and products made ofthis pulp (nonwovens or paper, especially tissue paper).

BACKGROUND ART

[0002] In the production of paper, we differentiate between chemicalpulp and mechanical pulp.

[0003] According to DIN 6730, chemical pulp is a fibrous materialobtained from plant raw materials from which most non-cellulosecomponents have been removed by chemical pulping without substantialmechanical post-treatment. In case of chemical pulping processes such asthe sulfite or sulfate (Kraft) process, primarily the lignin componentsand the hemi-cellulose components are dissolved from the wood to varyingdegrees depending on the field of application of the chemical pulp. Theresult is a fibrous material consisting primarily of cellulose.

[0004] Mechanical pulp is the general term for fibrous materials made ofwood entirely or almost entirely by mechanical means, optionally atincreased temperatures. Mechanical pulp is subdivided into the purelymechanical pulps (groundwood pulp and refiner mechanical pulp) as wellas mechanical pulps subjected to chemical pretreatment: chemo-mechanicalpulp (CMP), such as chemo-thermomechanical pulp (CTMP).

[0005] Chemical and mechanical pulp are also known by the generaldesignation pulp.

[0006] The strength of papers made from chemical pulps (hereinafterabbreviated as “strength of the chemical pulp”) is substantiallydetermined by the pulping degree of the wood used as starting material.The binding ability of the fibers first increases as more and morelignin is removed and reaches a maximum at a lignin content of about10%. At this residual lignin content, the fibrous material has goodstiffness and resistance to tearing along with a high binding ability.As delignification proceeds, the overall strength of the chemical pulpdecreases due to the strong attack of most of the pulping chemicals onthe carbohydrate portion (cellulose and hemicellulose) of the wood.

[0007] However, such extensive delignification is accompanied by lossesin yield so that, for many years, there have been efforts to developpulps having higher contents of residual lignin:

[0008] high-yield chemical pulps, according to DIN 6730 a chemical pulpprimarily pulped chemically, but subsequently subjected to mechanicaldefibration;

[0009] semi-chemical pulp, a pulp according to DIN 6730 obtained fromplant raw materials by chemical pulping from which only part of thenon-fibrous components have been removed and which is subjected tomechanical post treatment; and

[0010] chemi-mechanical pulps (CMP, CTMP, as defined above).

[0011] In the meantime, the recovery of primary fibers has focused, onthe one hand, on the production of mechanical pulps in very high yieldsby addition of 1% to 5% maximum of sodium sulfite such thathydrophilizing occurs by introducing sulfonic acid groups in conjunctionwith a low dissolving rate of the wood components, and on the otherhand, on the production of full chemical pulps according to the sulfateprocess.

[0012] If chemical pulp recovered by the sulfate process (called “Kraftpulp” or “sulfate chemical pulp”) is to be used for producing productswith a high degree of brightness, the lignin must be removed completely,since even small amounts of residual lignin are discolored due tochemical changes in the lignin structure (primary condensation). This islargely carried out in a pulping process. Any residual lignin stillpresent is then removed in several bleaching steps.

[0013] The native lignin is less discolored by the acidic sulfiteprocess. For this reason, it was possible to use high-yield chemicalpulps made by acidic or semi-acidic sulfite processes in unbleached formfor special applications. If a higher degree of brightness was required,such chemical pulps were also bleached in a delignifying manner withchlorine and bleaching agents containing chlorine. However, theseproducts were unable to satisfy the ever-increasing optical demands. Inaddition, environmental concerns about bleaching agents containingchlorine and chemical pulps containing chlorine led to a situation whereindustrial-level production of high-yield chemical pulps was notpursued. Furthermore, less expensive recycling paper has increasinglybeen used in paper production.

[0014] However, delignifying bleaching of chemical pulps often has thedisadvantage that chemicals used for bleaching also attack cellulosefibers to a considerable extent, thus reducing the strength of chemicalpulps. Therefore up until now, it has been thought that pulps ofsufficient strength and of a high degree of brightness could only beobtained by pulping to low kappa values followed by bleaching. This alsoapplies to the sulfite pulping process.

[0015] A summary of the development of acidic sulfite pulping processesfor preparing semi-chemical pulps is found in R. Runkel and K. F. Patt,“Halbzellstoffe” (Semichemical Pulps), Günther-Stalb Verlag, Biberach1958, pages 35 - 37 and pages 95 - 96. The production of high-yieldchemical pulps, semichemical pulps and chemi-mechanical pulps accordingto the sulfite process is also described in “S. A. Rydholm, PulpingProcesses, Interscience Publishers, New York, London, Sydney, 1965,pages 418 - 420”.

[0016] In addition, G. Jayme, L. Broschinksi, W. Matzke (in Das Papier18, 1964, pages 308 through 314) present a general survey of high-yieldchemical pulps and give a detailed description of rapid pulping in thevapor phase with magnesium bisulfite at a maximum temperature of 180° C.over a period of 8 to 20 minutes.

[0017] DE-A-1- 517 219 relates to the preparation of a (high-yield)sulfite chemical pulp. Wood raw material is pulped with an aqueoussolution containing sulfite and/or bisulfite ions as well as sodium,potassium, magnesium, or ammonium ions. The pH of the solutions at onsetof pulping is 3.0 to 7.0, preferably 3.7 to 5.0. The maximum pulpingtemperature is 140° to 190° C., preferably 150° to 170° C. The entirepulping process takes more than 400 minutes. The residence time at themaximum temperature is 30 to 200 minutes. Pulping is carried out at achlorine number of the finished chemical pulp in the range of 15 to 32,the pulped material then being subjected to controlled defibrationand/or defibration/refinement. After that, fines are removed in anamount of 0.2 to 7 % of the amount of the chemical pulp.

[0018] In unbeaten form (freeness value °SR =14.5 −15) the material thusobtained has a breaking length of 6.3 km. The chemical pulp is notbleached.

[0019] U.S. Pat. Nos.4,634,499 and 4,734,162 each relate to processesfor preparing a chemical pulp from hardwood which is especially suitablefor the preparation of tissue papers. Pulping is carried out withammonium sulfite, first at less than 110° C., then at a maximumtemperature of 140° C. to 155° C. at a pH of about 2 to 3. The chemicalpulp is not subjected to an additional bleaching step.

[0020] EP 0 287 960 A relates to a process for preparing a hemicellulosehydrolysate and a special chemical pulp by a two-step process, wherein

[0021] a first step comprises pre-hydrolysis of the ligno-cellulosicmaterials, for example, with water, a mineral acid, sulfur dioxide,sulfite pulping liquor, and sulfite waste liquor, at a temperature of100° to 180° C. and over a hydrolysis period of 10 to 200 minutes, and

[0022] a second step in which the lignin contained in the pre-hydrolysedmaterial is dissolved occurs by means of neutral sulfite pulping withaddition of anthraquinone as the catalyst, the initial pH being at least10. The temperature is preferably 160° to 180° C. and the treatment time100 to 200 minutes.

[0023] Following the second step, the resulting ligno-cellulosicmaterial is mechanically defibrated and optionally bleached.

[0024] Yields obtained in this manner range from 37.0 % to 45.7 % basedon the wood used, the kappa number ranges from 17.2 to 48. 1, and thedegree of brightness (ISO value) at 48.2 to 87.1.

[0025] GB-1,546,877 B relates to a CTMP (chemo-thermomechanical pulp)which is suitable for absorbant products such as tissue paper, and,additionally, has a light color. Yields at 85 % to 95 % are well abovethe common values for high yield pulps. The production process comprisesthe following steps:

[0026] washing of a ligno-cellulosic material with water

[0027] impregnation and pulping with a liquid containing sulfur dioxideat a temperature of 100° to 170° C., preferably at a pH of 5 to 11.

[0028] partial defibration of the resulting material by mechanical meanswith simultaneous bleaching.

[0029] Suitable bleaching chemicals are alkali metal peroxides such assodium peroxide or hydrogen peroxide, or reducing bleaching agents suchas dithionite, hydroxyl amine, thiourea, or thioglycolic acid.

[0030] An essential feature of the resulting chemical pulp is a contentof at least 10 % by weight of sulfonated fiber bundles, each consistingof two to four individual fibers.

[0031] This British patent does not address the strength properties ofthe chemical pulp obtained. The description of the patent further pointsto non-satisfactory degrees of brightness of the resulting chemicalpulp.

[0032] EP 0 446 110 A describes a process for bleaching chemical pulps(yield ranges approx. 85 to 90 %), which are obtained mechanically(mechanical pulp), optionally combined with chemical methods(chemomechanical pulp) and/or thermal methods. This bleaching processcomprises the following steps:

[0033] pretreatment of (mechanical or chemomechanical) pulp with acomplexing agent for metal ions followed by a washing step

[0034] treatment with sulfite and a more electronegative reducing agentat a pH between 7 and 12.5 followed by a washing step

[0035] bleaching treatment with hydrogen peroxide in an alkalineenvironment.

[0036] According to example 1, this process produces degrees ofbrightness of 83.9% ISO.

[0037] However, all processes known from the prior art for preparingpulps having a high degree of brightness have disadvantages. Inparticular, insufficient resistance against fiber collapse and poorstrength of individual fibers is observed in the pulp obtained. At lowdegrees of beating, known pulps often have strength values insufficientfor preparation of tissue products.

[0038] Bleached mechanical pulps and bleached chemomechanical pulps havethe further disadvantage of an unstable degree of brightness. This isdue to the fact that the bleaching chemicals convert chromophoric groupsof the lignin into non-chromophoric groups, which however are unstable.This type of reduction in the degree of brightness (discoloration) maybe induced either by light or heat.

[0039] A disadvantage of known chemical pulps is their chlorine contentand/or formation within the bleach of soluble reaction productscontaining chlorine, which is environmentally undesirable.

[0040] Therefore, it is the object of the present invention to makeavailable a pulp that has further properties in addition to a high anddurable optical quality, making it particularly suitable for theproduction of nonwovens and paper (products), in particular tissueproducts.

[0041] A further aspect of the invention is to make available a processfor producing such pulp.

[0042] Finally, a further object of the present invention is to makeavailable pulp based paper or nonwovens and/or products which arecharacterized by both resistance to mechanical stress and high qualityof appearance.

[0043] It is a further object of the invention to make available anenvironmentally compatible process for producing environmentallycompatible pulp and derivative paper and/or nonwoven products.

SUMMARY OF THE INVENTION

[0044] These objects are achieved by:

[0045] a process for producing a chlorine-free bleached sulfite chemicalpulp comprising the following steps:

[0046] delignification of chips (commonly woodchips) from ligncellulosicmaterial by a sulfite pulping process, in particular, in the presence ofmagnesium until the defibration point has been attained, and

[0047] bleaching of the resulting fibrous material in a chlorine-freebleaching sequence comprising at least one bleaching step using anoxidizing agent in the presence of a base;

[0048] chlorine-free bleached chemical pulp obtainable by a sulfitepulping process followed by chlorine-free bleaching, characterized inthat it has an ISO degree of brightness of at least 83 % and a strength,expressed as breaking length, of at least 6 km (measured according toZellcheming V/12/57), determined with a test sheet (ISO 5269-2, August1998) produced from the pulp without beating the same, and

[0049] paper and nonwovens obtainable from this pulp.

DETAILED DESCRIPTION OF THE INVENTION Process

[0050] In the first step of the process according to the invention,chips (commonly woodchips) from ligno-cellulosic material are partiallydelignified in a sulfite pulping process, in particular in the presenceof magnesium, until the point of defibration of the material isattained.

[0051] The point of defibration is the point in time during the pulpingat which the fibers subjected to chemical delignification may readily beseparated into individual fibers without mechanical defibration. Oncethe point of defibration has been reached, the individual fibers arecollected generally by pumping off the pulped lignocellulosic materialout of the cooker. The kappa value of the resulting pulp depends on thetype of wood, the degree of pulping, as well as on the chemical systemthat was applied. Preferably, the partially delignified fibrous materialobtained from the first step has a kappa value (according to DIN 54357,August 1978) of 50 to 75, in particular 60 to 70. This is the case, inparticular, for spruce or pine.

[0052] One advantage of the sulfite process used according to theinvention is that the point of defibration is obtained at considerablyhigher residual lignin contents and yields than in the sulfate pulpingprocess. This is due to the fact that in the initial pulping phase,sulfite solutions prefer to dissolve lignin in the middle lamella of thewood cells, and thereby the bonding between the wood cells is lost. Thisresults in a fiber surface of relatively low residual lignin content.The largest part of the yet remaining lignin is located in the centrallayer of the cell wall (S2) (on the structure of a wood cell, see e.g.FIG. 20.2, page 509 in T. P. Nevell and S. Haig Zeronian, CelluloseChemistry and Its Applications, John Wiley & Sons). In contrast to theresidual lignin content of sulfate pulp, the residual lignin content ofthe pulp resulting from the sulfite process, in particular the acidicsulfite process has not only relatively few chromophoric groups, but itis also less condensed and therefore more reactive.

[0053] The term “ligno-cellulosic material” includes all materialscontaining cellulose and lignin as main components, typically wood. In(dry) wood, the lignin content is generally at least 20 wt.-% (hardwoodabout 22 wt. %, conifers 27 - 33 wt.-%). Usually, the cellulose contentof wood is at least 40 wt.-%, generally 40 to 50 wt.-%. Both softwoodfrom conifers and hardwood from deciduous trees may be used in theprocess of the invention. Examples for suitable kinds of wood comprisespruce, pine, aspen, beech, birch, maple, poplar, and oak. In addition,eucalyptus is especially suitable as a source of fibers for tissuepapers. The use of spruce and beech is especially preferred.

[0054] Prior to pulping, the ligno-cellulosic material used is roughlychopped into chips. The size of the chips may vary and ranges forexample from 1 to 5 cm in breadth and length, with a thickness of up to1 cm.

[0055] Pulping (first step) is carried out with an acidic sulfitesolution in a known manner, e.g. with an aqueous solution containingsulfite and/or bisulfite ions as well as sodium, potassium, magnesium,or ammonium ions. Suitable methods are described in depth in G. A.Smook, M. J. Kocurek, Handbook for Pulp and Paper Technologists, Tappi,Atlanta, 1982, pages 58 through 65. Preferably, the chemical pulpingoccurs in a single step, i.e. without sudden (as contrast to gradual)changes in conditions of the process such as pH. The entire pulpingprocess does not require any additional mechanical means.

[0056] Chips of lignocellulosic material may be pulped in alkaline,neutral, or acidic conditions, however, preferably in an acidicenvironment with sulfite. Pulping at a pH of 1 to 5, especially 1.2 to4.0 is particularly preferred.

[0057] In a particularly preferred embodiment, pulping is performed withan aqueous solution containing sulfite and/or bisulfite ions as well asmagnesium ions. One advantage of this variant is the possibility ofrecovering MgO and SO₂ from the pulping waste liquors by thermaldecomposition and recycling them to the pulping process. The pulpingtemperature preferably ranges from 130° to 165° C., in particular 135°to 150° C. Until a maximum temperature of 130° to 150° C. is attained,heating should preferably be carried out over a period of 30 to 120minutes.

[0058] Then, the temperature is maintained at the maximum temperaturepreferably for a time period of 120 to 300 minutes. A time period of 30to 60 minutes is preferred for cooling from the maximum temperature(T_(max)) to room temperature.

[0059] Total pulping time (heating +pulping at T_(max)+cooling) rangespreferably from 180 to 480 minutes.

[0060] The SO₂ content of the pulping solution relative to the dryweight (see Examples for its determination) of the lignocellulosicmaterial used preferably equals 5 to 30 % by weight, in particular 15 to24 % by weight. The proportion of magnesium if present, relative to thedry weight of the lignocellulosic material, expressed as MgO, preferablyequals 4 to 10 % by weight, in particular 6 to 7.5 % by weight.

[0061] According to the invention, addition of further pulping chemicalssuch as, for example, anthraquinone is not required, but may be used inalkaline sulfite pulping processes.

[0062] The lignocellulosic material-to-liquor ratio preferably rangesfrom 1:3 to 1:5, especially from 1:3.5 to 1:3.7, relative to the dryweight of the lignocellulosic material.

[0063] The first step of the pulping process according to the inventionresults in an unbleached chemical pulp (hereinafter also referred to as“fibrous material”), which is then transferred to the bleachingsequence.

[0064] The yield of unbleached chemical pulp relative to the dry weightof the ligno-cellulosic material used is greater than 50 % by weight,preferably at least 55 % by weight, for example, about 60 % by weight.

[0065] The degree of brightness of the material thus obtained is usuallyequal to 35 to 60 % ISO, preferably 40 to 55 % ISO.

[0066] Preferably, the unbleached chemical pulp has the followingstrength parameters, each measured according to ISO 5269-2; August 1998,on test sheets having a basis weight of about 80 g/cm² using a standardclimate according to DIN EN 20187 (see description in the Examples). Allof the following values are based on unbeaten chemical pulp whichcorresponds to a freeness value (°SR, measured according to DIN-ISO5267/1; March 1999) of approx. 12 to 15:

[0067] a breaking length (dry, measured according to ZellchemingV/12/57) of at least 9 km, more preferably at least 10 km, especially 10to 11 km, and

[0068] a tear strength (dry, see Examples, measured according toElmendorff; DIN 53128) relative to 100 g/m², of at least 70 cN, morepreferably at least 75 cN, especially 85 to 100 cN.

[0069] The sulfite pulping process used as the first step of the processof the invention has the advantage over the sulfate process of producinga pulp having a relatively light color even at higher residual lignincontents.

[0070] This intermediate, which likewise is a part of the invention, aswell as the process steps producing it, is then subjected to bleachingin one or more steps.

[0071] Prior to bleaching, the chemical pulp resulting from the firststage (delignification) is separated from the cooking liquor in a knownmanner, e.g. it may be filtered and optionally washed (usually withwater).

[0072] The common feature of each bleaching sequence used according tothe invention is bleaching with a chlorine-free oxidant, in the presenceof a base in a so-called “first bleaching step”. The entire bleachingsequence is carried out with chlorine-free agents. The terms“chlorine-free bleaching sequence” and “chlorine-free bleachingchemicals” mean that the bleaching chemicals contain no chlorine,neither elemental chlorine, nor bonded chlorine such as, e.g. inchlorine dioxide.

[0073] The entire bleaching sequence is preferably carried out withoutadditional mechanical pulping means, i.e. defibration means.

[0074] The base used in the first bleaching step is preferably the samebase that is used in pulping. This makes it easier to close watercycles, since the filtrate from the washer after the first bleachingstep may be used for washing in the washer after the pulping step.Furthermore, by using the same base, preferably one containingmagnesium, the resulting bleach waste liquors may be disposed togetherwith the cooking waste liquors or at least partially recycled.

[0075] Sodium hydroxide, magnesium oxide (MgO), and/or magnesiumhydroxide (Mg(OH)₂) are preferred bases. Hydrogen peroxide (H₂O₂) is apreferred oxidant. The amount of oxidant is preferably 35 to 60 kg/t, inparticular 40 to 55 kg/t, relative to the dry weight of the fibrousmaterial used. The amount of base used generally ranges from 10 to 20kg/t relative to the dry weight of the fibrous material used.

[0076] The first bleaching step is preferably conducted at a temperatureof 60° to 80° C. The preferred pH range is from 8.5 to 9.5. The durationof this bleaching step is preferably 240 to 420 minutes.

[0077] The effect of this first bleaching step may be intensified byusing oxygen. Preferably oxygen is used in an amount of 0.5 % to 3 %relative to the dry weight of the fibrous material used.

[0078] This first bleaching step is preferably combined with at leastone other (so-called “second”) bleaching step which is also conductedwith an oxidant, in the presence of a base. In addition to the oxidantmentioned above (hydrogen peroxide), the use of peracetic acid (PAA),preferably in a pH range from 7 to 9, is especially suitable for thissecond optional bleaching step. The preferred base is sodium hydroxide(NaOH) or magnesium oxide (MgO) (where hereinafter the term “Magnesiumoxide (MgO)” also includes magnesium hydroxide (Mg(OH)₂). Both oxidantand base may be used in the same amounts set forth above. Duration andtemperature of this bleaching step may also correspond to those of thefirst bleaching step.

[0079] The advantage of a peracetic acid step (magnesium oxide as base)as the second optional bleaching step is the fact that additional wastewater cycles may be closed in order to minimize waste.

[0080] In particular, if using peroxides or peracids in the first orsecond (optional) bleaching step, bleaching abilities may be improved byfirst treating the fibrous material with a complexing agent. Accordingto the invention, examples of suitable complexing agents arenitrogen-containing organic compounds, in particular nitrogen-containingpolycarboxylic acids, nitrogen-containing polyphosphonic acids, andnitrogen-containing polyalcohols. Examples of nitrogen-containingpolycarboxylic acids are diethylenetriamine pentaacetic acid (DTPA),ethylenediamine tetraacetic acid (EDTA), and nitrilo-triacetic acid(NTA). Diethylenetriamine pentamethylenephosphonic acid (DTPMPA) anddiethylenetriamine pentaphosphonic acid are examples ofnitrogen-containing poylphosphonic acids. Furthermore, one may also useother complexing agents such as polycarboxylic acids, e.g. oxalic acid,citric acid or tartaric acid, or phosphonic acids. Preferred complexingagents are: DTPA, DTPMPA, and EDTA.

[0081] Preferably, complexing agent may be added in the amount of 0.5 to3 kg/t relative to the fibrous material used. Preferably, the complexingtreatment is performed at approx. 60° C.

[0082] In one embodiment, the complexing treatment is carried out in aweakly acidic environment (hereinafter referred to as “Q”).

[0083] In order to activate the residual lignin content of the fibrousmaterial, one may also use an oxidant in an acidic environment alongwith the complexing agent. For this purpose, peracetic acid (hereinafterreferred to as “Q/PAA”) and ozone are particularly suited.

[0084] In an additional, particularly preferred embodiment (referred toas an acidic wash or “A_(Q)”), one sets the pH of the pulp suspension ata value of 3 or less, e.g. by addition of H₂SO₄. In this manner, one maydissolve MgO that has precipitated onto the pulp or into the suspension.After setting the pH value, one adds the complexing agent to thesuspension. This form of complexing treatment is particularly suited asa first step of the bleaching sequence, or if it has been preceded by atleast one bleaching step in the presence of MgO as base.

[0085] The second (optional) bleaching step may be followed byadditional oxidizing bleaching steps with chlorine-free oxidants in thepresence of base. After treatment with peracetic acid, a third oxidizingbleaching step is preferably performed using hydrogen peroxide in thepresence of sodium hydroxide as base. This third oxidizing bleachingstep is preferably performed under the same conditions as thosedescribed in the first and second bleaching steps.

[0086] The oxidizing bleaching steps are preferably combined with aso-called “reducing bleaching step” using a reducing bleaching agent. Ina preferred embodiment, a reducing bleaching step concludes the entirebleaching sequence. In another preferred embodiment, the bleachingsequence comprises a first oxidative bleaching step, followed byreducing bleaching step, and then an additional oxidative bleachingstep. Bleaching agents that are suitable for the reducing bleaching stepinclude water-soluble dithionite salts, hydroxylamine, thiourea,thioglycolic acid, borohydride (e.g. sodium borohydride), orformamidinosulfinic acid. Especially preferred is the use offormamidinosulfinic acid or dithionite, particularly sodium dithionite.

[0087] The amount of reducing agent used in the reducing bleaching steppreferably ranges from 5 to 15 % by weight relative to the amount offibrous material used. This reaction is preferably performed at a pHranging from 9 to 11. Preferably, sodium hydroxide or magnesium oxide isused to set the pH. Magnesium oxide has the advantage that the waste ofthis step may be disposed together with pulping waste liquor. Thepreferred temperature for the reaction is in the range of 80° to 95° C.The reaction preferably takes from 60 to 90 minutes.

[0088] Selection and sequence of the bleaching steps may be varied,where with multi-step bleaching sequences it is preferred to begin thebleaching sequence with an oxidative bleaching of the typeP_(MgO)(bleaching step with hydrogen peroxide in the presence of MgO asbase), optionally in the presence of oxygen (OP_(MgO)), and to concludethe bleaching sequence with a reducing bleaching step.

[0089] The total residence time for all oxidizing and/or reducingbleaching steps (including complexing treatments) ranges preferably from700 to 1200 minutes.

[0090] Following each individual step of the bleaching sequence(including complexing treatments), the fibrous material is preferablyseparated from the bleaching solution, e.g. by filtration, and washed.In this manner, the consumption of chemicals in bleaching may often bereduced. By washing upon completion of the bleaching sequence, thepurity of the pulp according to the invention may be increased.Furthermore, it is preferred to work countercurrent, i.e. to wash thecompletely bleached fibrous material with clean water and to use theresulting wash water for pulp of the previous bleaching step or steps.Finally, the wash water from the washing following the first bleachingstep is preferably used for washing the pulp following the pulpingprocess. It is possible in the cases described above to use fresh waterin addition to the wash water that is led in countercurrent.

[0091] Preferred bleaching sequences are as follows:

[0092] 1) A_(Q) - (P_(MgO) or OP_(MgO)) - Q -P_(NaOH) -(FAS_(NaOH) orY),

[0093] 2) A_(Q) - (P_(MgO) or OP_(MgO)) -Q -PAA_(NaOH) -P_(NaOH)-(FAS_(NaOH) or Y),

[0094] 3) Q - (P_(MgO) or OP_(MgO)) - Q -PAA_(MgO) -P_(NaOH)-(FAS_(NaOH) or Y),

[0095] 4) A_(Q) - (p_(NaOH) or OP_(NaOH)) - Q - P_(NaOH) - (FAS_(NaOH)or Y),

[0096] 5) Q/PAA - (P_(MgO) or OP_(MgO)) - Q/PAA - FAS_(MgO) - P_(NaOH),or

[0097] 6) Q - P_(MgO) - A_(Q) - P_(NaOH) - FAS_(NaOH)

[0098] 7) Q - P_(MgO) - Q - P_(NaOH) - FAS_(NaOH)

[0099] symbols meaning the following: A_(Q) acid wash with complexingtreatment Q complexing treatment Q/PPA complexing treatment withconcurrent activation of fiber surface with peracetic acid P_(NaOH/MgO)bleaching step with hydrogen peroxide in presence of NaOH or MgO as baseOP_(NaOH/MgO) bleaching step with hydrogen peroxide in the presence ofoxygen and NaOH or MgO as base PAA_(NaOH/MgO) bleaching step withperacetic acid in the presence of NaOH or MgO as base FAS_(NaOH/MgO)reducing bleaching step with formamidinosulfinic acid in the presence ofNaOH or MgO. Y reducing bleaching step with dithionite.

[0100] Based on the most recent knowledge, bleaching sequences 6 and 7represent the best mode for practicing the invention.

Pulp

[0101] In the manner described above, one may obtain bleached chemicalpulp according to the invention. This pulp has the properties of achemical pulp that is obtained by a sulfite pulping process, inparticular a magnesium (bi)sulfite process, and that is then bleachedwithout the use of chlorine or chlorine-containing chemicals. It has abrightness of at least 83% ISO and is further characterized by abreaking length (dry) of at least 6 km, preferably at least 7 km, morepreferred at least 8 km, in particular at least 9 km, e.g. 10 km[measured (according to Zellcheming V/12/57) on a test sheet made fromunbeaten pulp (freeness of 12 to 15° SR measured according to DIN-ISO5267/1; March 1999) basis weight of approx. 80 g/m², produced accordingto ISO 5269-2; August 1998, in a standard climate according to DIN EN20187; November 1993)].

[0102] The degree of brightness is preferably at least 84 % ISO,particularly at least 85% ISO. For example, the degree of brightness maybe 86 % ISO.

[0103] One advantage of the pulp according to the invention is that incontrast to mechanical pulp or chemomechanical pulp, its degree ofbrightness does not significantly decline during further processing.

[0104] The pulp according to the invention is substantially free ofchlorine and/or chloride Preferably, the pulp has an OX content of lessthan 30 mg/kg, or in particular, is free of OX. The OX content relatesto the residual halogen organic compounds in the pulp, which mayessentially be formed during bleaching and are measured in accordancewith DIN 38414/18 and PTS-RHO12/90.

[0105] Pulp that is low in OX or is OX-free, and/or products derivedfrom such pulp are more environmentally compatible than pulp andpulp-derived products containing OX. This also applies to the productionprocess. In order to close water cycles as much as possible, it islikewise preferred that only chlorine-free chemicals be used in pulpproduction, because that way a build-up of chlorine, chloride, and/orchlorine-containing substances may be avoided.

[0106] These properties of the pulp make it especially suitable for theproduction of tissue paper.

[0107] Preferably, (bleached, unbeaten) pulp according to the inventionhas

[0108] a tear strength (dry, measured according to Elmendorff (DIN53128) on test sheets described above under breaking length) relative to100 g/m² of at least 90 cN, more preferably at least 95 to 105 cN.

[0109] The (bleached) pulp according to the invention preferably has avery low fiber bundle content of bundles having at least two fibers,i.e. preferably less than 5 % by weight, in particular less than 1 % byweight relative to the dry weight. It is more preferred for it to befree of such bundles.

[0110] In a preferred embodiment, the pulp is ground following thebleaching sequence. Preferably, it has a freeness value of more than 15,in particular more than 15 and simultaneously not more than 18 °SR. Withan increased freeness value there is an increase in breaking length.

[0111] This additional surface treatment (beating), which has afavorable effect on the strength properties of the resultingpaper/tissue paper, may preferably be brought about within the pulprefinement system of a paper/tissue paper machine. In another preferredembodiment, such surface treatment (beating) occurs as part of pulpproduction, i.e. while it is still at the pulp plant. A refiner isparticularly suitable for this purpose. Fibrillation of the surfaceoccurs during mechanical treatment of the pulp/water suspension. Thistreatment influences the static and dynamic strength properties.

[0112] Depending on the refiner's operating mode, the fibers areshortened (cut) or are fibrillated, which includes the separation of theouter layers of the fiber wall, this latter process substantiallyincreasing the surface and bonding capacity of the fibers. The refineroperating mode associated with fibrillation is therefore preferred.

[0113] The residual lignin content of the bleached pulp according to theinvention (measured according to DIN 54357, August 1978) rangespreferably from 10 to 30, more preferred from 15 to 25, in particularfrom 18 to 22. One may estimate the residual lignin content bymultiplying the kappa numbers by 0.15. A kappa number of 20, which lieswithin the inventive range, thus corresponds to a residual lignincontent of approx. 3 % by weight.

[0114] The process according to the invention thus makes it possible toa large extent to delignify lignocellulosic material without the aid ofmechanical pulping means.

[0115] The average fiber length of the inventive bleached pulp rangespreferably from 1.8 to 2.2 mm (measured according to Kajaani using aKajaani machine FS-200).

[0116] The water retention capacity of the pulp according to theinvention ranges preferably from 130 to 150 %, in particular from 140 to145 % (measured according to Zellcheming IV/33/57, as described in theExamples).

Paper or Nonwoven (Product)

[0117] The present invention also relates to paper or nonwovencomprising the bleached pulp according to the invention, preferably inthe amount of at least 50 % by weight, in particular at least 80 % byweight, relative to the dry weight of the finished product.

[0118] The paper can be a packaging paper, a graphic paper or tissuepaper. Preferably, the paper is a tissue paper.

[0119] The tissue paper or nonwoven may be one-ply or multiple-ply.

[0120] The German terms “Vlies” and “Vliesstoffe” are applied to a widerange of products which in terms of their properties are located betweenthe groups, paper, paperboard, and cardboard on the one hand and thetextile products on the other, and are currently summarized under theterm “nonwovens” (see ISO 9092 - EN 29092). The invention allows theapplication of known processes for producing nonwovens, such as what arecalled air-laid and spun-laid techniques, as well as wet-laidtechniques.

[0121] Nonwovens may also be called textile-like composite materials,which represent flexible porous fabrics that are not produced by theclassic methods of weaving warp and weft or by looping, but byintertwining and/or by cohesive and/or adhesive bonding of fibers whichmay for example be present in the form of endless fibers orprefabricated fibers of a finite length, as synthetic fibers produced insitu or in the form of staple fibers. The nonwovens according to theinvention may thus consist of mixtures of synthetic fibers in the formof staple fibers and the pulp according to the invention.

[0122] “Papers” are also planar materials, albeit essentially composedof fibers of a plant origin and formed by drainage of a fibrous-materialsuspension on a wire or between two continuously revolving wires and bysubsequent compression and drainage or drying of the thus producedfibrous mat (cf. DIN 6730, May 1996). The standard restricts the rangeof mass per unit area (basis weight) for paper to a maximum of 225 g/m².

[0123] Depending on the type of paper, the production process comprisealso a sizing and/or smoothing step, along with the typical processsteps of sheet formation, pressing, and drying described above.

[0124] Based on the underlying compatibility of the production processes(wet laying), “tissue” production is counted among the paper makingtechniques. The production of tissue is distinguished from paperproduction by its extremely low basis weight of normally less than 40g/m² and its much higher tensile energy absorption index. (In processinginventive pulp to tissue paper, one generally selects a basis weight of8 to 65 g/m², especially 10 to 40 g/m². The total basis weight ofmultiple-ply tissue products is preferably equal to a maximum of 65g/m².) The tensile energy absorption index is arrived at from thetensile energy absorption in which the tensile energy absorption isrelated to the test sample volume before inspection (length, width,thickness of sample between the clamps before tensile load). Paper andtissue paper also differ in general with regard to the modulus ofelasticity that characterizes the stress-strain properties of theseplanar products as a material parameter.

[0125] A tissue's high tensile energy absorption index results from theouter or inner creping. The former is produced by compression of thepaper web adhering to a dry cylinder as a result of the action of acrepe doctor or in the latter instance as a result of a difference inspeed between two wires (“fabrics”). This causes the still moist,plastically deformable paper web to be internally broken up bycompression and shearing, thereby rendering it more stretchable underload than an uncreped paper. Most of the functional properties typicalof tissue and tissue products result from the high tensile energyabsorption index (see DIN EN 12625-4 and DIN EN 12625-5).

[0126] One example of papers and paper products is represented byhygiene papers, particularly tissue papers and hygiene products (tissueproducts) made therefrom and which are e.g. used in personal groomingand hygiene, the household sector, industry, the institutional field ina wide variety of cleaning processes. They are used to absorb fluids,for decorative purposes, for packaging or even as supporting material,as is common for example in medical practices or in hospitals.

[0127] Hygiene paper primarily includes all kinds of dry-creped tissuepaper, as well as wet-creped paper and cellulose or pulp wadding.

[0128] The one-ply intermediate products originating from thepaper-making machine and made of lightweight paper usually dry-creped ona yankee cylinder by means of a crepe doctor are generally described as“tissue paper” or more accurately raw tissue paper. The one-ply rawtissue may be built up of one or a plurality of layers respectively.

[0129] All one-ply or multi-ply final products made of raw tissue andtailored to the end user's needs, i.e. fabricated with a wide variety ofrequirements in mind, are known as “tissue products”.

[0130] Typical properties of tissue paper include the ready ability toabsorb tensile stress energy, their drapability, good textile-likeflexibility, properties which are frequently referred to as bulksoftness, a high surface softness, a high specific volume with aperceptible thickness, as high a liquid absorbency as possible and,depending on the application, a suitable wet and dry strength as well asan interesting visual appearance of the outer product surface. Theseproperties allow tissue paper to be used for example as cleaning cloths,sanitary products (e.g. toilet paper), paper handkerchiefs, cosmeticwipes (facials) or as serviettes/napkins.

[0131] If tissue paper is to be made out of the pulp according to theinvention, the process essentially comprises

[0132] a forming that includes the headbox and the wire portion,

[0133] b the drying portion (TAD (through air drying) or conventionaldrying on the yankee cylinder) that also usually includes the crepeprocess essential for tissues,

[0134] c the monitoring and winding area.

[0135] Paper can be formed by placing the fibers, in an oriented orrandom manner, on one or between two continuously revolving wires of apaper making machine while simultaneously removing the main quantity ofwater of dilution until dry-solids contents of usually between 12 and 35% are obtained.

[0136] Drying the formed primary fibrous web occurs in one or more stepsby mechanical and thermal means until a final dry-solids content ofusually about 93 to 97 %. In the case of tissue making, this stage isfollowed by the crepe process which crucially influences the propertiesof the finished tissue product in conventional processes. Theconventional dry crepe process involves creping on a usually 4.5 to 6 mdiameter drying cylinder, the so-called yankee cylinder, by means of acrepe doctor with the aforementioned final dry-solids content of the rawtissue paper (wet creping can be used if lower demands are made of thetissue quality). The creped, finally dry raw tissue paper (raw tissue)is then available for further processing into the paper product ortissue paper product according to the invention.

[0137] Instead of the conventional tissue making process describedabove, the invention gives preference to the use of a modified techniquein which an improvement in specific volume is achieved by a special kindof drying within process section b and in this way an improvement in thebulk softness of the thus made tissue paper is achieved. This process,which exists in a variety of subtypes, is termed the TAD (through airdrying) technique. It is characterized by the fact that the “primary”fibrous web (like a nonwoven) that leaves the sheet making stage ispre-dried to a dry-solids content of about 80% before final contactdrying on the yankee cylinder by blowing hot air through the fibrousweb. The fibrous web is supported by an air-permeable wire or belt andduring its transport is guided over the surface of an air-permeablerotating cylinder drum. Structuring the supporting wire or belt makes itpossible to produce any pattern of compressed zones broken up bydeformation in the moist state, resulting in increased mean specificvolumes and consequently leading to an increase in bulk softness withoutdecisively decreasing the strength of the fibrous web.

[0138] Another possible influence on the softness and strength of theraw tissue lies in the production of a layering in which the primaryfibrous web to be formed is built up by a specially constructed headboxin the form of physically different layers of fibrous material, theselayers being jointly supplied as a pulp strand to the sheet makingstage.

[0139] When processing the raw fibrous web or raw tissue paper into thefinal product (third process section), the following procedural stepsare normally used individually or in combination: cutting to size(longitudinally and/or cross cutting), producing a plurality of plies,producing mechanical ply adhesion, volumetric and structural embossing,ply adhesion, folding, imprinting, perforating, application of lotions,smoothing, stacking, rolling up.

[0140] To produce multi-ply tissue paper products, such ashandkerchiefs, toilet paper, towels or kitchen towels, an intermediatestep preferably occurs with so-called doubling in which the raw tissuein the finished product's desired number of plies is usually gathered ona common multiply master roll.

[0141] The processing step from the raw tissue that has already beenoptionally wound up in several plies to the finished tissue productoccurs in processing machines which include operations such as repeatedsmoothing of the tissue, edge embossing, to an extent combined with fullarea and/or local application of adhesive to produce ply adhesion of theindividual plies (raw tissue) to be combined together, as well aslongitudinal cut, folding, cross cut, placement and bringing together aplurality of individual tissues and their packaging as well as bringingthem together to form larger surrounding packaging or bundles. Theindividual paper ply webs can also be pre-embossed and then combined ina roll gap according to the foot-to-foot or nested methods.

EXAMPLES

[0142] In the description of the invention and in the following examplesthe following test methods were used to evaluate the chemical pulpsaccording to the invention.

1) Producing the Test Sheets

[0143] The test sheets (having a basis weight of approx. 80 g/m²) weremade in accordance with ISO 5269-2: August 1998). Before being tested interms of physical properties e.g. by means of the tensile test, the thusobtained test sheets were always conditioned for a duration of at least12 hours in a standard climate at a temperature of (23 ±1)° C. and arelative humidity of (50±2)% in accordance with DIN EN 20187; November1993, paper, cardboard and pulp, a standard climate for pretreatment andtesting and a method of monitoring the climate and pretreatment ofsamples, November 1993 (see ISO 187 1990).

2) Breaking Length (Dry)

[0144] The breaking length was determined using a dry test sheetproduced according to 1) following a process described in ZellchemingV/12/1957.

3) Tear Strength (Dry) according to Elmendorff

[0145] Tear strength was determined according to Elmendorff using a testsheet produced according to 1) following a process described in DIN53128.

4) Kappa Number

[0146] The kappa number is determined according to DIN 54357 (August1978)

5) WRV (Water Retention Value)

[0147] The WRV is determined in the following way.

[0148] The principle of determining the WRV is based on centrifugingswollen pulp samples at room temperature in a special-purpose centrifugebeaker with 3000-fold gravitational acceleration. The WRV in percent(mass portions in percent) indicates the portion of water in the samplethat cannot be centrifuged.

[0149] The implementation followed the German Zellcheming Code ofPractice IV/33/57 (issued on Jan. 1, 1957)

[0150] A fibrous material treated with excess water as a swelling agentis taken.

[0151] The tubes for the centrifuge inserts are filled to about 2/3 ofthe volume with the moist pulp sample without pressing firmly.

[0152] A triple determination is to be performed each time.

[0153] The tubes are inserted into the centrifuge beaker.

[0154] Centrifuging conditions: 15 min at 4800 rpm

[0155] After the centrifuge comes to a standstill, the tubes are takenout and the centrifuged substance immediately transferred with the aidof pincers as completely as possible into the weighing bottlespreviously dried at 103° C. and tared at room temperature where they areweighed (remove glass spheres beforehand)

[0156] The samples are now dried for at least 6 hours in thecirculating-air drying cabinet, immediately seal when taking them outand allow them to cool in the desiccator. Now reweigh.

[0157] The calculation is made on the basis of the following equation:

WRV =^((W-D))x 100

[0158] D

[0159] where W =the weight of the moist substance, D represents the dryweight of the substance; and W minus D =the weight of the absorbedwater.

6) Dry Weight

[0160] The weight values that are given as “dry weight” relate to driedmaterial, lignocellulosic material, pulp, paper, or nonwoven (product)that was dried at 105° C. until constant weight was achieved.

7) Freeness Value

[0161] The freeness value (in °SR) was measured according to DIN-ISO5267/1; March 1999.

8) Degree of Brightness

[0162] The degree of brightness (in %) was determined according to ISOfollowing SCAN C11:1975.

Example 1

[0163] Pine chips of a width of 10 to 25 mm, a length of 13 to 35 mm andwith a thickness of 1 to 10 mm were pulped in a magnesium bisulfitesolution (free SO₂ as hydrogen sulfite or sulfite-bound SO₂) having atotal SO₂ content of 23.6 % by weight/wood (dry weight) and a magnesiumcontent measured as MgO of 7 % by weight/wood (dry weight) at an initialpH of 1.7 and temperature of 138° C. The wood-to-liquor ratio was 1:3.5.The warming to the maximum temperature of 138° C. took 30 minutes. Thetemperature was maintained at the maximum level for a total of 4.5 hours(270 minutes). Once the defibration point was reached, cooling to roomtemperature required 60 minutes. The resulting pulp was separated fromwaste liquor, washed with water, and dried, yielding 58.2 % (dry weight)relative to the wood used, at a kappa number of 52.2 and a degree ofbrightness of 46.1 % ISO. The breaking length of the unbeaten (=freenessvalue of 14° SR), unbleached pulp was 10.3 km at a tear strengthaccording to Elmendorff of 87 cN relative to 100 g/m².

[0164] The washed pulp subsequently was subjected to complexingtreatment, being contacted at a temperature of 60° C. for a period of 30minutes with 2 kg/t of complexing agent DTPA at a density of 3 % (unlessotherwise stated, the data given in kg/t in Examples 1 and 2consistently refers to the dry weight of pulp). The pH value was set at5.2 for the complexing treatment with H₂SO₄. Then the pulp was bleachedin aqueous solution containing 50 kg/t of hydrogen peroxide and 15 kg/tMgO (at a density of 10 %) for a reaction time of 360 minutes and at areaction temperature of 70° C. This was followed by an additionalcomplexing treatment under the same conditions as described earlier, andsubsequently by a second oxidizing bleaching with 50 kg/t hydrogenperoxide and 22.5 kg/t sodium hydroxide (at a density of 10 %). A totalof 50 kg/t of peroxide was consumed in the two steps.

[0165] Finally, the resulting pulp was bleached for the last time,reduced with formamidinosulfinic acid (FAS). In the FAS step, 5 kg/tformamidinosulfinic acid was used at a temperature of 90° C. and densityof 10 % for a period of 60 minutes.

[0166] Following treatment with complexing agents and each bleachingstep, the pulp was washed with water.

[0167] The degree of brightness of the resulting pulp was recorded at85.9 % ISO. The yield (dry weight) of the pulp obtained after thebleaching sequence was 95.2 % by weight relative to the dry weight ofthe pulp subjected to the bleaching sequence. The bleached pulp had abreaking length of 9 km and a tear strength according to Elmendorff of102.4 cN relative to 100 g/m² in the unbeaten state (freeness value=15°SR).

Example 2

[0168] Pine chips, 10 to 25 mm wide, 13 to 35 mm long, and 2 to 10 mmthick, were pulped in a magnesium bisulfite solution containing 16 % byweight total SO₂/wood (dry weight) and 6.2 % by weight MgO/wood (dryweight). The wood(dry weight)-to-liquor ratio was 1:3.7. Warming to themaximum temperature of 150° C. took 60 minutes, and the pulping at thistemperature lasted 270 min (4.5 h). Cooling to room temperature occurredfor a period of 60 minutes. The initial pH of the cooking liquor was3.5. The yield of pulp after separation from the waste liquor, washingwith water, and drying was 58.9 %, at a kappa number of 60.2 and degreeof brightness of 47.8 % ISO. The unbeaten (freeness value =14.5° SR),unbleached pulp had a breaking length of 10.8 km and a tear strengthaccording to Elmendorff of 96.8 cN relative to a weight of 100 g/m².

[0169] This pulp was subjected to the same bleaching sequence as that inExample 1, and attained a degree of brightness of 83.3% ISO. A total of38 kg/t peroxide was consumed in this bleaching sequence. The yield (dryweight) after the bleaching sequence relative to the yield of pulp (dryweight) after pulping was 95.6 %. In the unbeaten state (freeness value15°SR), the bleached pulp had a breaking length of 9.4 km and a tearstrength according to Elmendorff of 99.5 cN relative to 100 g/m².

[0170] The pulp, pulped and bleached according to the invention, and/orany paper produced from such are characterized by an excellentcombination of degree of brightness and strength properties. In contrastto other pulp having similar residual lignin contents, the pulpaccording to the invention has a high stability in degree of brightness.

1. A process for preparing bleached sulfite chemical pulp, comprising the following steps: delignifying chips of a lignocellulosic material in a sulfite pulping process until a defibration point of said lignocellulosic material is reached, and a fibrous material is obtained; and bleaching the fibrous material with a bleaching sequence which is performed exclusively with chlorine-free reagents and comprises at least a first bleaching step with a chlorine-free oxidant in the presence of a base.
 2. The process according to claim 1, wherein the fibrous material has a kappa value ranging between 50 and
 75. 3. The process according to claim 1, wherein the sulfite pulping is conducted under acidic conditions.
 4. The process according to claim 1, wherein the sulfite pulping is conducted in the presence of magnesium in an acidic environment.
 5. The process according to claim 1, wherein the sulfite pulping is conducted with an aqueous solution containing magnesium bisulfite, optionally an excess of SO₂, at a temperature of 130° to 165° C. and at a pH of 1.5 to
 4. 6. The process according to claim 1, wherein the oxidant is hydrogen peroxide.
 7. The process according to claim 1, wherein the reaction sequence for chlorine-free bleaching of pulp comprises at least one additional bleaching step with a chlorine-free oxidant in the presence of a base.
 8. The process according to claim 7, wherein the oxidant used in the additional bleaching step is hydrogen peroxide or peractic acid.
 9. The process according to claim 1, wherein prior to treatment with an oxidant, treatment with a metal ion complexing agent is conducted.
 10. The process according to claim 1, wherein the reaction sequence for chlorine-free bleaching of pulp comprises a final bleaching step with a reducing bleaching agent.
 11. The process according to claim 10, wherein the reducing bleaching agent is selected from the group consisting of a water-soluble dithionite salt, hydroxylamine, thio urea, thioglycolic acid, borohydride hydride, and formamidinosulfinic acid.
 12. A chemical pulp, obtainable by a sulfite pulping process and subsequent chlorine-free bleaching, wherein said chemical pulp has a degree of brightness of at least 83% ISO and a test sheet produced from said chemical pulp without beating has a strength, expressed as breaking length (measured according to Zellcheming V/12/57), of at least 6 km.
 13. The chemical pulp according to claim 12, wherein said chemical pulp has a kappa number of between 10 and
 30. 14. The chemical pulp according to claim 12, wherein said chemical pulp is subjected to a beating step following bleaching.
 15. A paper or nonwoven prepared from a chemical pulp according to claim
 12. 16. A paper according to claim 15 which is tissue paper.
 17. A product prepared from nonwoven or paper according to claim
 15. 18. The process according to claim 6, wherein the reaction sequence for chlorine-free bleaching of pulp comprises at least one additional bleaching step with a chlorine-free oxidant in the presence of a base.
 19. The process according to claim 18, wherein the oxidant used in the additional bleaching step is hydrogen peroxide or peractic acid.
 20. The process according to claim 6, wherein prior to treatment with an oxidant, treatment with a metal ion complexing agent is conducted.
 21. The process according to claim 6, wherein the reaction sequence for chlorine-free bleaching of pulp comprises a final bleaching step with a reducing bleaching agent.
 22. The process according to claim 10, wherein the reducing bleaching agent is selected from the group consisting of a water-soluble dithionite salt, hydroxylamine, thio urea, thioglycolic acid, borohydride hydride, and formamidinosulfinic acid. 